While the invention is subject to a wide range of applications, it is especially suited for use in a hydraulic circuit for actuating double-acting motors and will be particularly described in that connection. A typical device of this type is an open-center valve which, together with a reservoir, a pump, and a double-acting differential area piston motor, form a control system for positioning the earth-moving bucket of a front-end loader. Frequently, these valves have four operative positions, namely: a "raised" position in which the pump and reservoir are connected with opposite sides of the fluid motor for raising the bucket; a "power-down" position in which the pump and reservoir connections with the fluid motor are reversed for lowering the bucket; a "neutral" or "hold" position in which the connection with each side of the fluid motor is blocked and the pump is unloaded to the reservoir to maintain the bucket in a fixed position; and a "float" position in which both sides of the fluid motor are placed in communication with the pump and an unloading path is established between the pump and the reservoir. In the float position, two types of operation are provided. In one type, the bucket floats down from the raised position without cavitation and faster than the power-down rate. At this time, the pump flow is available for other functions. The other operation permits the bucket being moved across the ground to float up and down according to the contour of the ground.
A hollow spool design is very beneficial for a control valve including the float position. For instance, as the bucket floats down, more fluid may leave the head end of the motor than is drawn into the rod end. Loss of fluid in the rod end can cause cavitation. By connecting the cylinder ports together and bleeding off the excess flow to tank, a positive pressure is maintained at the cylinder ports to drive oil back to the rod end and thereby eliminate cavitation. During the movement of the hollow spool valve from the raise position to the hold position, fluid can pass from one side of the motor into the reservoir. This results in the bucket dropping from its desired position. It is a well-known expedient, as shown in U.S. Pat. No. 2,994,346 to Ruhl to place a load check valve in the internal flow passage of the valve to prevent this backflow.
When the front loader is used for grading, the power-down position is used to lower the bucket. This may actually cause the wheels of the machine to be raised from the ground. The weight of the machine puts a large load on the motor cylinder. When the desired position of the bucket is reached, the control valve is moved to the hold position. During this movement, high pressure fluid from the cylinder can pass into the reservoir and permit undesired movement of the bucket. Since it is beneficial to provide a hollow spool valve for the float position, a load check valve as used in the raise position, cannot be incorporated in the control valve as it eliminates the float position.
A need to provide a hollow spool control valve with a float position and a load check valve for both the raise and power-down positions is recognized. A new load check valve which can satisfy this need may also be used for applications other than in a control valve.
It is an object of the present invention to provide a hydraulic control valve which includes an anti-backflow check valve for the power-down position.
It is a further object of the present invention to provide a hydraulic control valve which includes a float position without cavitation.
It is a further object of the present invention to provide a hydraulic control valve which has increased ease of operation.
It is a further object of the present invention to provide a hydraulic control valve which is relatively easy to assemble.
It is a further object of the present invention to provide a load check valve which can direct flow to either an outlet or a bypass opening.